Three-quarters of the way through this book, after a futuristic opening about the colonisation of a new planet, and an interlude from the fictional diaries of a shipwrecked member of Captain Cook's Easter Island crew, there are a few pages of what feels like authentic Jeanette Winterson autobiography, of the kind she made so wonderfully her own in Oranges Are Not the Only Fruit. Suddenly, a digressive, intermittently philosophical piece of science fiction based upon a love story between a woman and a female robot finds an authentic heart. This kernel of what sounds like truth-telling among all the doomy fabrication is almost unbearably sad. It also seems to show Winterson gaining insight into what makes her write her intensely driven fiction.

The fragment is a reverie of her alter ego in the novel, Billie Crusoe, a lone voice of humanity on a blighted planet. Crusoe is thinking of voids, of how they might be filled, and of home, where it may lie. She meditates on her arrival into the world, beached and unwanted: 'My mother's body split open and I was the cargo for salvage. I suppose you have to believe there was something worth salvaging and with me it seems that nobody did.'

She believes she can feel that first month of her life inside her, abandoned by a juvenile mother, touted door to door by a desperate grandmother, eventually given up for adoption. She believes, or wants to believe, she can remember her mother coming searching for her, a face at the window of the orphanage, the first planet she sees from her cot, a wailing figure at the end of the street - 'smaller and smaller, like a light-years-away star?' - the source of the unquestioning love she has always yearned for, never discovered. That search has been Billie Crusoe's subconscious quest. 'You never stop looking. That's what I found, though it took me years to know that's what I have been doing ... I live an echo of another life.'

That shadow existence has, Winterson seems to suggest here, always haunted her and has given her a compulsion for storytelling. The author mentioned in a recent interview that, during the course of writing this novel, she discovered her adoption papers for the first time and the fact of them hit her with the force of revelation. She writes here of Billie Crusoe's 'childhood, which has somehow turned up again, like an orphan on my doorstep asking to be let in'.

The emotional insistence of this intruder gives proper shape and purpose to a narrative that constantly threatens to come adrift. Winterson has often seemed to want to make a universe of her own emotion, most obviously in her book-length take on Marvell's To HisCoy Mistress, Written on the Body,and here she makes that case explicitly. Billie Crusoe's lost mother becomes a metaphor for humanity's destruction of the planet and loss of its home. The book is set in an apocalyptic near-future that has been brought about by the obvious forces of desolation. 'While we were all arguing about whether we should fly less, drive less, eat less, weigh less, consume less, dump less, carbon dioxide in the atmosphere rose to 550 parts per million, the icecaps melted and Iran launched a nuclear attack on the USA.'

What's left is Tech City, an autocratic state policed by robots and populated by genetically modified humans fixed in eternal youth. While the population will never get old, the planet is geriatric and dying. The only hope is love, which for Billie Crusoe comes in the unlikely form of her impossibly beautiful 'robo sapiens' protege, Spike; the pair of them fetch up at a derelict Jodrell Bank and hear the voices of a distant planet, and set off to discover a new home.

Winterson uses this vague take on Defoe as an opportunity for no end of poetic parables about the rape of the planet and man's inhumanity to man. Her blunt satire takes on some obvious targets, the modern tyrannies of parking meters and cosmetic surgery and corporate autocracy for a start, but her attempts at making a Ballard-style dystopia are never really convincing.

The novel, like much of Winterson's recent writing, asks to be read as a powerfully felt eco-treatise as much as a piece of fiction - until, that is, her sudden personal intervention takes it somewhere else entirely.

· Jeanette Winterson will be in conversation with Alex Clark at the Bloomsbury Theatre, London WC2 on Thursday 11 October, 7pm. For details call 0845 456 9876; observer.theguardian.com/writerstalks